Towards accurate dose accumulation for Step-&-Shoot IMRT: Impact of weighting schemes and temporal image resolution on the estimation of dosimetric motion effects

Statistical breathing curve sampling to quantify interplay effects of moving lung tumors in a 4D Monte Carlo dose calculation framework

PURPOSE

The interplay between respiratory tumor motion and dose application by intensity modulated radiotherapy (IMRT) techniques can potentially lead to undesirable and non-intuitive deviations from the planned dose distribution. We developed a 4D Monte Carlo (MC) dose recalculation framework featuring statistical breathing curve sampling, to precisely simulate the dose distribution for moving target volumes aiming at a comprehensive assessment of interplay effects.

METHODS

We implemented a dose accumulation tool that enables dose recalculations of arbitrary breathing curves including the actual breathing curve of the patient. This MC dose recalculation framework is based on linac log-files, facilitating a high temporal resolution up to 0.1 s. By statistical analysis of 128 different breathing curves, interplay susceptibility of different treatment parameters was evaluated for an exemplary patient case. To facilitate prospective clinical application in the treatment planning stage, in which patient breathing curves or linac log-files are not available, we derived a log-file free version with breathing curves generated by a random walk approach. Interplay was quantified by standard deviations σ in D_5%, D_50% and D_95%.

RESULTS

Interplay induced dose deviations for single fractions were observed and evaluated for IMRT and volumetric arc therapy (σ_D95% up to 1.3 %) showing a decrease with higher fraction doses and an increase with higher MU rates. Interplay effects for conformal treatment techniques were negligible (σ<0.1%). The log-file free version and the random walk generated breathing curves yielded similar results (deviations in σ< 0.1 %) and can be used as substitutes for interplay assessment.

CONCLUSION

It is feasible to combine statistically sampled breathing curves with MC dose calculations. The universality of the presented framework allows comprehensive assessment of interplay effects in retrospective and prospective clinically relevant scenarios.

Reduction of breathing irregularity-related motion artifacts in low-pitch spiral 4D CT by optimized projection binning

Background

Respiration-correlated CT (4D CT) is the basis of radiotherapy treatment planning of thoracic and abdominal tumors. Current clinical 4D CT images suffer, however, from artifacts due to unfulfilled assumptions concerning breathing pattern regularity. We propose and evaluate modifications to existing low-pitch spiral 4D CT reconstruction protocols to counteract respective artifacts.

Methods

The proposed advanced reconstruction (AR) approach consists of two steps that build on each other: (1) statistical analysis of the breathing signal recorded during CT data acquisition and extraction of a patient-specific reference breathing cycle for projection binning; (2) incorporation of an artifact measure into the reconstruction. 4D CT data of 30 patients were reconstructed by standard phase- and local amplitude-based reconstruction (PB, LAB) and compared with images obtained by AR. The number of artifacts was evaluated and artifact statistics correlated to breathing curve characteristics.

Results

AR reduced the number of 4D CT artifacts by 31% and 27% compared to PB and LAB; the reduction was most pronounced for irregular breathing curves.

Conclusions

We described a two-step optimization of low-pitch spiral 4D CT reconstruction to reduce artifacts in the presence of breathing irregularity and illustrated that the modifications to existing reconstruction solutions are effective in terms of artifact reduction.

Electronic supplementary material

The online version of this article (doi:10.1186/s13014-017-0835-7) contains supplementary material, which is available to authorized users.

4D dose simulation in volumetric arc therapy: Accuracy and affecting parameters

Radiotherapy of lung and liver lesions has changed from normofractioned 3D-CRT to stereotactic treatment in a single or few fractions, often employing volumetric arc therapy (VMAT)-based techniques. Potential unintended interference of respiratory target motion and dynamically changing beam parameters during VMAT dose delivery motivates establishing 4D quality assurance (4D QA) procedures to assess appropriateness of generated VMAT treatment plans when taking into account patient-specific motion characteristics. Current approaches are motion phantom-based 4D QA and image-based 4D VMAT dose simulation. Whereas phantom-based 4D QA is usually restricted to a small number of measurements, the computational approaches allow simulating many motion scenarios. However, 4D VMAT dose simulation depends on various input parameters, influencing estimated doses along with mitigating simulation reliability. Thus, aiming at routine use of simulation-based 4D VMAT QA, the impact of such parameters as well as the overall accuracy of the 4D VMAT dose simulation has to be studied in detail–which is the topic of the present work. In detail, we introduce the principles of 4D VMAT dose simulation, identify influencing parameters and assess their impact on 4D dose simulation accuracy by comparison of simulated motion-affected dose distributions to corresponding dosimetric motion phantom measurements. Exploiting an ITV-based treatment planning approach, VMAT treatment plans were generated for a motion phantom and different motion scenarios (sinusoidal motion of different period/direction; regular/irregular motion). 4D VMAT dose simulation results and dose measurements were compared by local 3% / 3 mm γ-evaluation, with the measured dose distributions serving as ground truth. Overall γ-passing rates of simulations and dynamic measurements ranged from 97% to 100% (mean across all motion scenarios: 98% ± 1%); corresponding values for comparison of different day repeat measurements were between 98% and 100%. Parameters of major influence on 4D VMAT dose simulation accuracy were the degree of temporal discretization of the dose delivery process (the higher, the better) and correct alignment of the assumed breathing phases at the beginning of the dose measurements and simulations. Given the high γ-passing rates between simulated motion-affected doses and dynamic measurements, we consider the simulations to provide a reliable basis for assessment of VMAT motion effects that–in the sense of 4D QA of VMAT treatment plans–allows to verify target coverage in hypofractioned VMAT-based radiotherapy of moving targets. Remaining differences between measurements and simulations motivate, however, further detailed studies.

Comparison of 3D and 4D Monte Carlo optimization in robotic tracking stereotactic body radiotherapy of lung cancer

PURPOSE

To investigate the adequacy of three-dimensional (3D) Monte Carlo (MC) optimization (3DMCO) and the potential of four-dimensional (4D) dose renormalization (4DMCrenorm) and optimization (4DMCO) for CyberKnife (Accuray Inc., Sunnyvale, CA) radiotherapy planning in lung cancer.

MATERIALS AND METHODS

For 20 lung tumors, 3DMCO and 4DMCO plans were generated with planning target volume (PTV5 mm) = gross tumor volume (GTV) plus 5 mm, assuming 3 mm for tracking errors (PTV3 mm) and 2 mm for residual organ deformations. Three fractions of 60 Gy were prescribed to ≥ 95 % of the PTV5 mm. Each 3DMCO plan was recalculated by 4D MC dose calculation (4DMCrecal) to assess the dosimetric impact of organ deformations. The 4DMCrecal plans were renormalized (4DMCrenorm) to 95 % dose coverage of the PTV5 mm for comparisons with the 4DMCO plans. A 3DMCO plan was considered adequate if the 4DMCrecal plan showed ≥ 95 % of the PTV3 mm receiving 60 Gy and doses to other organs at risk (OARs) were below the limits.

RESULTS

In seven lesions, 3DMCO was inadequate, providing < 95 % dose coverage to the PTV3 mm. Comparison of 4DMCrecal and 3DMCO plans showed that organ deformations resulted in lower OAR doses. Renormalizing the 4DMCrecal plans could produce OAR doses higher than the tolerances in some 4DMCrenorm plans. Dose conformity of the 4DMCrenorm plans was inferior to that of the 3DMCO and 4DMCO plans. The 4DMCO plans did not always achieve OAR dose reductions compared to 3DMCO and 4DMCrenorm plans.

CONCLUSION

This study indicates that 3DMCO with 2 mm margins for organ deformations may be inadequate for Cyberknife-based lung stereotactic body radiotherapy (SBRT). Renormalizing the 4DMCrecal plans could produce degraded dose conformity and increased OAR doses; 4DMCO can resolve this problem.

Estimation of lung motion fields in 4D CT data by variational non-linear intensity-based registration: A comparison and evaluation study

Accurate and robust estimation of motion fields in respiration-correlated CT (4D CT) images, usually performed by non-linear registration of the temporal CT frames, is a precondition for the analysis of patient-specific breathing dynamics and subsequent image-supported diagnostics and treatment planning. In this work, we present a comprehensive comparison and evaluation study of non-linear registration variants applied to the task of lung motion estimation in thoracic 4D CT data. In contrast to existing multi-institutional comparison studies (e.g. MIDRAS and EMPIRE10), we focus on the specific but common class of variational intensity-based non-parametric registration and analyze the impact of the different main building blocks of the underlying optimization problem: the distance measure to be minimized, the regularization approach and the transformation space considered during optimization. In total, 90 different combinations of building block instances are compared. Evaluated on proprietary and publicly accessible 4D CT images, landmark-based registration errors (TRE) between 1.14 and 1.20 mm for the most accurate registration variants demonstrate competitive performance of the applied general registration framework compared to other state-of-the-art approaches for lung CT registration. Although some specific trends can be observed, effects of interchanging individual instances of the building blocks on the TRE are in general rather small (no single outstanding registration variant existing); the same level of accuracy is, however, associated with significantly different degrees of motion field smoothness and computational demands. Consequently, the building block combination of choice will depend on application-specific requirements on motion field characteristics.

Dose-escalation study for cardiac radiosurgery in a porcine model

PURPOSE

To perform a proof-of-principle dose-escalation study to radiosurgically induce scarring in cardiac muscle tissue to block veno-atrial electrical connections at the pulmonary vein antrum, similar to catheter ablation.

METHODS AND MATERIALS

Nine mini-pigs underwent pretreatment magnetic resonance imaging (MRI) evaluation of heart function and electrophysiology assessment by catheter measurements in the right superior pulmonary vein (RSPV). Immediately after examination, radiosurgery with randomized single-fraction doses of 0 and 17.5-35 Gy in 2.5-Gy steps were delivered to the RSPV antrum (target volume 5-8 cm(3)). MRI and electrophysiology were repeated 6 months after therapy, followed by histopathologic examination.

RESULTS

Transmural scarring of cardiac muscle tissue was noted with doses ≥32.5 Gy. However, complete circumferential scarring of the RSPV was not achieved. Logistic regressions showed that extent and intensity of fibrosis significantly increased with dose. The 50% effective dose for intense fibrosis was 31.3 Gy (odds ratio 2.47/Gy, P<.01). Heart function was not affected, as verified by MRI and electrocardiogram evaluation. Adjacent critical structures were not damaged, as verified by pathology, demonstrating the short-term safety of small-volume cardiac radiosurgery with doses up to 35 Gy.

CONCLUSIONS

Radiosurgery with doses >32.5 Gy in the healthy pig heart can induce circumscribed scars at the RSPV antrum noninvasively, mimicking the effect of catheter ablation. In our study we established a significant dose-response relationship for cardiac radiosurgery. The long-term effects and toxicity of such high radiation doses need further investigation in the pursuit of cardiac radiosurgery for noninvasive treatment of atrial fibrillation.

Multivariate regression approaches for surrogate-based diffeomorphic estimation of respiratory motion in radiation therapy

Breathing-induced location uncertainties of internal structures are still a relevant issue in the radiation therapy of thoracic and abdominal tumours. Motion compensation approaches like gating or tumour tracking are usually driven by low-dimensional breathing signals, which are acquired in real-time during the treatment. These signals are only surrogates of the internal motion of target structures and organs at risk, and, consequently, appropriate models are needed to establish correspondence between the acquired signals and the sought internal motion patterns. In this work, we present a diffeomorphic framework for correspondence modelling based on the Log-Euclidean framework and multivariate regression. Within the framework, we systematically compare standard and subspace regression approaches (principal component regression, partial least squares, canonical correlation analysis) for different types of common breathing signals (1D: spirometry, abdominal belt, diaphragm tracking; multi-dimensional: skin surface tracking). Experiments are based on 4D CT and 4D MRI data sets and cover intra- and inter-cycle as well as intra- and inter-session motion variations. Only small differences in internal motion estimation accuracy are observed between the 1D surrogates. Increasing the surrogate dimensionality, however, improved the accuracy significantly; this is shown for both 2D signals, which consist of a common 1D signal and its time derivative, and high-dimensional signals containing the motion of many skin surface points. Eventually, comparing the standard and subspace regression variants when applied to the high-dimensional breathing signals, only small differences in terms of motion estimation accuracy are found.

Dosimetric evaluation of four-dimensional dose distributions of CyberKnife and volumetric-modulated arc radiotherapy in stereotactic body lung radiotherapy

Advanced image‐guided stereotatic body lung radiotherapy techniques using volumetric‐modulated arc radiotherapy (VMAT) with four‐dimensional cone‐beam computed tomography (4D CBCT) and CyberKnife with real‐time target tracking have been clinically implemented by different authors. However, dosimetric comparisons between these techniques are lacking. In this study, 4D CT scans of 14 patients were used to create VMAT and CyberKnife treatment plans using 4D dose calculations. The GTV and the organs at risk (OARs) were defined on the end‐exhale images for CyberKnife planning and were then deformed to the midventilation images (MidV) for VMAT optimization. Direct 4D Monte Carlo dose optimizations were performed for CyberKnife (4DCK). Four‐dimensional dose calculations were also applied to VMAT plans to generate the 4D dose distributions (4DVMAT) on the exhale images for direct comparisons with the 4DCK plans. 4DCK and 4DVMAT showed comparable target conformity (1.31±0.13 vs. 1.39±0.24,p=0.05). GTV mean doses were significantly higher with 4DCK. Statistical differences of dose volume metrics were not observed in the majority of OARs studied, except for esophagus, with 4DVMAT yielding marginally higher D1% than 4DCK. The normal tissue volumes receiving 80%, 50%, and 30% of the prescription dose (V80%,V50%, and V30%) were higher with 4DVMAT, whereas 4DCK yielded slightly higher V10% in posterior lesions than 4DVMAT. VMAT resulted in much less monitor units and therefore greater delivery efficiency than CyberKnife. In general, it was possible to produce dosimetrically acceptable plans with both techniques. The selection of treatment modality should consider the dosimetric results as well as the patient's tolerance of the treatment process specific to the SBRT technique.

PACS numbers: 87.53.Ly, 87.55.km

Advances in 4D radiation therapy for managing respiration: part I - 4D imaging

Techniques for managing respiration during imaging and planning of radiation therapy are reviewed, concentrating on free-breathing (4D) approaches. First, we focus on detailing the historical development and basic operational principles of currently-available "first generation" 4D imaging modalities: 4D computed tomography, 4D cone beam computed tomography, 4D magnetic resonance imaging, and 4D positron emission tomography. Features and limitations of these first generation systems are described, including necessity of breathing surrogates for 4D image reconstruction, assumptions made in acquisition and reconstruction about the breathing pattern, and commonly-observed artifacts. Both established and developmental methods to deal with these limitations are detailed. Finally, strategies to construct 4D targets and images and, alternatively, to compress 4D information into static targets and images for radiation therapy planning are described.